High surface quality GaN wafer and method of fabricating same

ABSTRACT

Al x Ga y In z N, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1, characterized by a root mean square surface roughness of less than 1 nm in a 10×10 μm 2  area. The Al x Ga y In z N may be in the form of a wafer, which is chemically mechanically polished (CMP) using a CMP slurry comprising abrasive particles, such as silica or alumina, and an acid or a base. High quality Al x Ga y In z N wafers can be fabricated by steps including lapping, mechanical polishing, and reducing internal stress of said wafer by thermal annealing or chemical etching for further enhancement of its surface quality. CMP processing may be usefully employed to highlight crystal defects of an Al x Ga y In z N wafer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 10/272,761filed Oct. 17, 2002, now allowed, which in turn is acontinuation-in-part of U.S. patent application Ser. No. 09/877,437filed Jun. 8, 2001 in the names of Xueping Xu and Robert P. Vaudo,issued Dec. 3, 2002 as U.S. Pat. No. 6,488,767.

GOVERNMENT RIGHTS IN INVENTION

The invention disclosed herein includes aspects that were involved inthe performance of United States Contract No. DASG60-00-C-0036 issued bythe U.S. Army Space and Missile Defense Command and United StatesContract No. N00014-00-3-0013 issued by The Office of Naval Research.The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to Al_(x)Ga_(y)In_(z)N (wherein 0≦x≦1, 0≦y≦1,0≦z≦1, and x+y+z=1) having superior surface quality, including invarious embodiments, articles formed of such Al_(x)Ga_(y)In_(z)Nmaterial, e.g., in wafer form, including surfaces comprisingcrystallographic plane surfaces and offcuts of such crystallographicplane surfaces that are suitable for fabrication of microelectronic andoptoelectronic device structures. The invention also relates to methodsfor fabricating such Al_(x)Ga_(y)In_(z)N articles and surfaces.

2. Description of the Related Art

GaN and related GaN-like III-V nitride crystal films, represented by thegeneral formula Al_(x)Ga_(y)In_(z)N, where 0≦x≦1, 0≦y≦1, 0≦z≦1, andx+y+z=1, are useful materials in various applications, such as hightemperature electronics, power electronics, and optoelectronics (e.g.,light emitting diodes (LEDs) and blue light laser diodes (LDs)). Bluelight emitting diodes (LEDs) and lasers are an enabling technology,allowing much higher storage density in magneto-optic memories andCDROMs and the construction of full color light emitting displays. Bluelight emitting diodes may replace currently employed incandescent lightbulbs in road and railway signals etc., since in such applications bluelight emitting diodes have the potential to achieve very substantialcost and energy savings.

Currently, Al_(x)Ga_(y)In_(z)N films are grown on non-native substratessuch as sapphire or silicon carbide, due to unavailability of highquality Al_(x)Ga_(y)In₂N substrates. However, differences in thermalexpansion and lattice constants between such foreign substrates and theAl_(x)Ga_(y)In_(z)N crystals epitaxially grown thereon cause significantthermal stress and internal stress in the grown Al_(x)Ga_(y)In_(z)Ncrystals. The thermal stress and internal stress cause micro-cracks,distortions, and other defects in the Al_(x)Ga_(y)In_(z)N crystals, andmake such Al_(x)Ga_(y)In_(z)N crystals easy to break. Growing on latticenon-matched foreign substrates also causes high density of latticedefects, leading to poor device performance. In order to reduce thedeleterious thermal stress and high defect density in the grownAl_(x)Ga_(y)In_(z)N crystals, it is desirable to provide high qualityfreestanding Al_(x)Ga_(y)In_(z)N wafers as film-growing substrates, inplace of the above-mentioned foreign substrates. U.S. Pat. No. 5,679,152entitled “Method for Making a Single Crystal Ga*N Article” and U.S.patent application No. application Ser. No. 08/955,168 filed Oct. 21,1997 entitled “Bulk Single Crystal Gallium Nitride and Method of MakingSame” disclose hydride vapor phase epitaxy (HVPE) processes forfabricating freestanding Al_(x)Ga_(y)In_(z)N crystals as substrates forhomoepitaxial growth of Al_(x)Ga_(y)In_(z)N single crystal materialthereon.

Since quality of a subsequently grown Al_(x)Ga_(y)In_(z)N crystal isdirectly correlated to the quality of the substrate surface and nearsurface region on which the Al_(x)Ga_(y)In_(z)N crystal is grown, it isimportant to provide a highly smooth initial substrate surface withoutany surface or subsurface damage.

However, after mechanical polishing, Al_(x)Ga_(y)In_(z)N crystalstypically have very poor surface quality, with substantial surface andsubsurface damage and polishing scratches. Additional wafer finishprocessing therefore is necessary to further enhance the surface qualityof the freestanding Al_(x)Ga_(y)In_(z)N crystal, so that it is suitablefor high-quality epitaxial growth and device fabrication thereon.

Crystalline Al_(x)Ga_(y)In_(z)N generally exists in a chemically stablewurtzite structure. The most common crystallographic orientation ofAl_(x)Ga_(y)In_(z)N compounds has two polar surfaces perpendicular toits c-axis: one side is N-terminated, and the other one is Ga-terminated(Ga hereinafter in the context of the Ga-side of the crystal structurebeing understood as generally illustrative and representative ofalternative Group III (Al_(x)Ga_(y)In_(z)) crystalline compositions,e.g., of a corresponding Ga_(x)In_(y)-side in Ga_(x)In_(y)N crystals, ofa corresponding Al_(x)Ga_(y)In_(z)-side in Al_(x)Ga_(y)In_(z)N crystals,of a corresponding Al_(x)Ga_(y)-side in Al_(x)Ga_(y)N crystals, etc.).

Crystal polarity strongly influences the growth morphology and chemicalstability of the crystal surface. It has been determined that the N-sideof the Al_(x)Ga_(y)In_(z)N crystal is chemically reactive with KOH orNaOH-based solutions, whereas the Ga-side of such crystal is very stableand not reactive with most conventional chemical etchants. The N-sidecan therefore be easily polished, using an aqueous solution of KOH orNaOH, to remove surface damage and scratches left by the mechanicalpolishing process and to obtain a highly smooth surface.

The Ga-side (Al_(x)Ga_(y)In_(z) side) of the Al_(x)Ga_(y)In_(z)Ncrystal, on the other hand, remains substantially the same aftercontacting the KOH or NaOH solution, with its surface damage andscratches unaltered by such solution. See Weyher et al., “ChemicalPolishing of Bulk and Epitaxial GaN”, J. CRYSTAL GROWTH, vol. 182, pp.17-22, 1997; also see Porowski et al. International Patent ApplicationPublication No. WO 98/45511 entitled “Mechano-Chemical Polishing ofCrystals and Epitaxial Layers of GaN and Ga_(1-x-y)Al_(x)In_(y)N”.

However, it has been determined that the Ga-side of theAl_(x)Ga_(y)In_(z)N crystal is a better film-growing surface than theN-side. See Miskys et al., “MOCVD-Epitaxy on Free-Standing HVPE-GaNSubstrates”, PHYS. STAT. SOL. (A), vol. 176, pp. 443-46, 1999. Ittherefore is important to provide a wafer finish process that isparticularly effective for preparing the Ga-side of theAl_(x)Ga_(y)In_(z)N crystal to make it suitable for subsequent crystalgrowth thereupon.

Reactive ion etching (RIE) recently has been used to remove a layer ofsurface material from the Ga-side of an Al_(x)Ga_(y)In_(z)N wafer toobtain smoother wafer surface. See Karouta et al., “Final Polishing ofGa-Polar GaN Substrates Using Reactive Ion Etching”, J. ELECTRONICMATERIALS, vol. 28, pp. 1448-51, 1999. However, such RIE process isunsatisfactory because it is ineffective for removing deeper scratches,and it introduces additional damage by ion bombardment and additionalsurface irregularities by concomitant contamination, which in turnrequires additional cleaning of the GaN wafer in an O₂ plasma.

It is therefore advantageous to provide an Al_(x)Ga_(y)In_(z)N waferwith high surface quality on its Ga-side, with substantially no orlittle surface and subsurface damage or contamination. It is alsodesirable that such Al_(x)Ga_(y)In_(z)N wafer is prepared by a surfacepolishing process that is both economic and effective, and requires nocumbersome cleaning process during or after polishing.

More generally, even though there is a particular need in the art for asurface polishing process that produces high surface quality on theAl_(x)Ga_(y)In_(z)-terminated side of the Al_(x)Ga_(y)In_(z)N(0001)substrate, since such Al_(x)Ga_(y)In_(z)-terminated surface is the mostchemically stable surface, there is also a continuing need in the artfor Al_(x)Ga_(y)In_(z)N wafer articles with high surface quality onother crystallographic surfaces and offcuts of such surfaces, e.g.,non-polar a-axis surfaces, N-terminated (0001) surfaces, A-planesurfaces, M-plane surfaces, R-plane surfaces, and offcuts of theforegoing surfaces.

SUMMARY OF THE INVENTION

The present invention generally relates to Al_(x)Ga_(y)In_(z)N (wherein0≦y≦1 and x+y+z=1) having superior surface quality, including in variousembodiments, device fabrication surfaces comprising crystallographicplane surfaces and offcuts of such crystallographic plane surfaces, andto methods of fabricating such Al_(x)Ga_(y)In_(z)N material in waferform with surfaces suitable for microelectronic and/or optoelectronicdevice manufacture.

One aspect of the present invention relates to a high qualityAl_(x)Ga_(y)In_(z)N wafer having a surface roughness characterized by aroot means square (RMS) roughness of less than 1 nm in a 10×10 μm² area,e.g., at its Ga-side, at its N-side, at offcuts of (0001) surfaces, atA-plane surfaces, at M-plane surfaces, at R-plane surfaces, at offcutsof A-plane surfaces, at offcuts of M-plane surfaces, and/or at offcutsof R-plane surfaces.

Although described hereinafter with illustrative reference to Ga-sidesurfaces of Al_(x)Ga_(y)In_(z)N articles, it will be understood that thepolished surface articles of the invention, and the chemical mechanicalpolishing compositions and their methods of use, broadly encompass andrelate to surfaces of Al_(x)Ga_(y)In_(z)N other than theAl_(x)Ga_(y)In_(z)-terminated side of Al_(x)Ga_(y)In_(z)N(0001)articles, such as the N-terminated side of Al_(x)Ga_(y)In_(z)N(0001)articles, and offcuts of (0001) surfaces, as well as A-, M- and R-planesurfaces, and offcuts of such respective A-, M- and R-plane surfaces.

Accordingly, subsequent references to Ga-side surfaces ofAl_(x)Ga_(y)In_(z)N articles in the disclosure hereof will be understoodas being representative of and alternatively applicable to such othersurfaces, e.g., N-terminated surfaces of Al_(x)Ga_(y)In_(z)N(0001)articles, offcuts of (0001) surfaces of Al_(x)Ga_(y)In_(z)N(0001)articles, as well as A-, M- and R-plane surfaces of Al_(x)Ga_(y)In_(z)Narticles, and offcuts of such respective A-, M- and R-plane surfaces ofAl_(x)Ga_(y)In_(z)N articles.

The invention provides Al_(x)Ga_(y)In_(z)N articles of superior RMSsurface roughness characteristics, such as Al_(x)Ga_(y)In_(z)N wafershaving a surface, e.g., the exemplary Ga-side surface, useful forfabrication of microelectronic devices, optoelectronic devices, orcorresponding device precursor structures.

In ranges of progressively increasing preference, the RMS surfaceroughness of such wafer, e.g., at its Ga-side, is within the followingranges: (1) less than 0.7 nm in a 10×10 μm² area; (2) less than 0.5 nmin a 10×10 μm² area; (3) less than 0.4 nm in a 2×2 μm² area; (4) lessthan 0.2 nm in a 2×2 μm² area; and (5) less than 0.15 nm in a 2×2 μm²area.

Al_(x)Ga_(y)In_(z)N wafers according to the present invention preferablyare characterized by a regular step structure at the Ga-side thereofwhen observed by atomic force microscope.

Al_(x)Ga_(y)In_(z)N wafers according to the present invention preferablyare characterized by the crystal defects of the Al_(x)Ga_(y)In_(z)Nwafer at its Ga-side constituting small pits with diameters of less than1 μm. Small pits of such size are readily visible by both atomic forcemicroscope (AFM) and scanning electron microscope (SEM) techniques,while at the same time these pits do not constitute significant damageof the Al_(x)Ga_(y)In_(z)N wafer surface and therefore do not impairquality of Al_(x)Ga_(y)In_(z)N crystals subsequently grown thereon.

Such high quality Al_(x)Ga_(y)In_(z)N crystal wafers are readilymanufactured by chemically mechanically polishing (CMP) ofAl_(x)Ga_(y)In_(z)N wafer blanks, using silica or alumina-containing CMPslurry compositions. The corresponding CMP process enables the crystaldefects of the Al_(x)Ga_(y)In_(z)N wafer (evidenced by small pits ofless than 1 μm in diameter) to be readily visualized.

Another aspect of the present invention relates to an epitaxialAl_(x)Ga_(y)In_(z)N crystal structure, comprising an epitaxialAl_(x′)Ga_(y′)In_(z′)N (wherein 0≦y′≦1 and x′+y′+z′=1) film grown on theabove-described Al_(x)Ga_(y)In_(z)N wafer of the invention. Suchepitaxial Al_(x′)Ga_(y′)In_(z′)N crystal structure preferably comprisesa wurtzite crystalline thin film, but may be in any other suitable formor structure suitable for specific semiconductor, electronic, oroptoelectronic applications. The composition of the epitaxial film maybe or may not be the same as the composition of the wafer substrate. Theepitaxial Al_(x′)Ga_(y′)In_(z′)N crystal structure may comprise severalepitaxial Al_(x′)Ga_(y′)In_(z′)N films with different composition ordoping, sequentially grown on the above-described Al_(x)Ga_(y)In_(z)Nwafer of the invention. The epitaxial film may have graded composition,i.e., the composition of the epitaxial film varies with the distancefrom the interface between the substrate and epitaxial film. As usedherein, the term “thin film” means a material layer having a thicknessof less than about 100 μm.

Yet another aspect of the present invention relates to an optoelectronicdevice that comprises at least one such epitaxial Al_(x′)Ga_(y′)In_(z′)Ncrystal structure grown on the above-described Al_(x)Ga_(y)In_(z)N waferof the invention.

A further aspect of the present invention relates to a microelectronicdevice that comprises at least one such epitaxial Al_(x′)Ga_(y′)In_(z′)Ncrystal structure grown on the above-described Al_(x)Ga_(y)In_(z)N waferof the invention.

A further aspect of the present invention relates to anAl_(x)Ga_(y)In_(z)N boule that comprises epitaxial Al_(x)Ga_(y)In_(z)Ncrystal structure grown on the above-described Al_(x)Ga_(y)In_(z)N waferof the invention. A boule is defined as a bulk crystal body that can besliced into at least two wafers. An Al_(x)Ga_(y)In_(z)N boule can begrown with any suitable method such as hydride vapor phase epitaxy(HVPE), the metallorganic chloride (MOC) method, metallorganic chemicalvapor deposition (MOCVD), sublimation, liquid phase growth, etc.

The invention in a further aspect contemplates a method of chemicallymechanically polishing an Al_(x)Ga_(y)In_(z)N wafer, using a CMP slurrycomprising:

-   -   abrasive amorphous silica particles, e.g., having an average        particle size of less than 200 nm;    -   at least one acid; and    -   optionally, at least one oxidation agent;        wherein the pH value of the CMP slurry may be of a suitable        character for the polishing operation, e.g., an acidic pH value        (0 to <7.0).

The abrasive amorphous silica particles in the CMP slurry may forexample comprise fumed silica or colloidal silica. The amorphous silicaparticles in the CMP slurry in one embodiment of the present inventionhave an average particle size in the range from about 10 nm to about 100nm. The CMP slurry of the invention in various embodiments thereof caninclude one or more oxidation agent(s), e.g., hydrogen peroxide,dichloroisocyanuric acid, or the like.

The pH value of such CMP slurry may be varied in differing embodimentsof the invention, and in particular embodiments the CMP slurry may beformulated so that its pH is in a specific range of values, e.g., arange of 0≦pH<7, a range of from about 0.6 to about 3, a range of fromabout 0.5 to about 4, or a range of from about 0.8 to about 2.5, invarious respective embodiments.

In one such illustrative embodiment, the CMP slurry comprises an acidicchloro silica slurry.

A further aspect of the present invention relates to a method ofchemically mechanically polishing an Al_(x)Ga_(y)In_(z)N wafer, using aCMP slurry comprising:

-   -   abrasive colloidal alumina particles, e.g., having an average        particle size of less than 200 nm;    -   at least one acid; and    -   optionally, at least one oxidation agent;        wherein the pH value of the CMP slurry is of a suitable        character for the polishing operation, e.g., an acidic pH value        (0 to <7.0).

In one illustrative embodiment of such aspect of the invention, the pHof the CMP slurry composition is in a range of from about 3 to about 5.In another illustrative embodiment, the pH of the CMP slurry compositionis in a range of from about 3 to about 4.

The abrasive colloidal alumina particles in the CMP slurry may vary inparticle size in differing embodiments of the invention. In oneembodiment of the invention, the CMP slurry has an average particle sizein a range of from about 10 nm to about 100 nm.

The above-described colloidal alumina CMP slurry of the invention can invarious embodiments include one or more oxidation agent(s), e.g.,hydrogen peroxide, dichloroisocyanuric acid, or the like.

A further aspect of the present invention relates to chemical mechanicalpolishing (CMP) of the Al_(x)Ga_(y)In_(z)N wafer, using a CMP slurrythat comprises:

-   -   amorphous silica particles, e.g., having an average particle        size of less than 200 nm;    -   at least one base; and    -   optionally, at least one oxidation agent,        wherein the pH value of the CMP slurry is in a basic pH range        (pH>7.0).

The amorphous silica particles in such CMP slurry in one embodiment ofthe invention comprise fumed silica particles having an average particlesize in a range of from about 10 nm to about 100 nm, and in anotherembodiment the amorphous silica particles comprise colloidal silicaparticles having an average particle size in a range of from about 10 nmto about 100 nm.

Bases useful for the practice of such CMP compositional aspect of thepresent invention include, but are not limited to, ammonia,alkanolamines, and hydroxides, e.g., KOH or NaOH. Ammonia andalkanolamines are particularly preferred, since they also function tostabilize the CMP slurry.

Such CMP slurry may include one or more oxidation agent(s), e.g.,hydrogen peroxide, dichloroisocyanuric acid or the like.

The pH value of such basic amorphous silica CMP slurry may be widelyvaried in the practice of the invention. In one illustrative embodiment,the pH is in a range of from about 7 to about 14. In anotherillustrative embodiment, the pH is in a range of from about 8 to about13.5. In a further embodiment, the pH is in a range of from about 9 toabout 13. In yet another embodiment, the pH is in a range of from about10 to about 11.

A further aspect of the present invention relates to a method ofhighlighting crystal defects of an Al_(x)Ga_(y)In_(z)N wafer, e.g., atits Ga-side, to facilitate determination of crystal defect density ofsuch wafer, comprising the steps of:

-   -   providing an Al_(x)Ga_(y)In_(z)N wafer;    -   chemically mechanically polishing the wafer, according to one of        the above-described CMP methods of the invention;    -   cleaning and drying the polished Al_(x)Ga_(y)In_(z)N wafer; and    -   scanning the wafer with an atomic force microscope or a scanning        electron microscope to determine defect density in the wafer.

In one aspect of the invention, the CMP process is conducted using anacidic silica slurry as described hereinabove.

Yet another aspect of the present invention relates to a method offabricating high quality Al_(x)Ga_(y)In_(z)N wafers, comprising thesteps of:

-   -   providing an Al_(x)Ga_(y)In_(z)N wafer blank having thickness in        a range of from about 100 μm to about 1000 μm;    -   optionally reducing internal stresses of the Al_(x)Ga_(y)In_(z)N        wafer blank;    -   optionally lapping the Al_(x)Ga_(y)In_(z)N wafer blank at a        first side thereof, using a lapping slurry comprising abrasives        having an average particle size in a range of from about 5 μm to        about 15 μm;    -   optionally mechanically polishing the Al_(x)Ga_(y)In_(z)N wafer        blank at such first side thereof, using a mechanical polishing        slurry comprising abrasives having average particle size in a        range of from about 0.1 μm to about 6 μm;    -   optionally lapping the Al_(x)Ga_(y)In_(z)N wafer blank at a        second side thereof, using a lapping slurry comprising abrasives        having an average particle size in a range of from about 5 μm to        about 15 μm;    -   mechanically polishing the Al_(x)Ga_(y)In_(z)N wafer blank at        such second side thereof, using a mechanical polishing slurry        comprising abrasives having average particle size in a range of        from about 0.1 μm to about 6 μm;    -   chemically mechanically polishing the Al_(x)Ga_(y)In_(z)N wafer        blank at such second side thereof, using a CMP slurry comprising        at least one chemical reactant and abrasive colloidal particles        having an average particle size of less than 200 nm, to produce        a corresponding Al_(x)Ga_(y)In_(z)N wafer; and    -   optionally mild etching to further reduce internal stresses of        the Al_(x)Ga_(y)In_(z)N wafer and improve the surface quality,        wherein the resultant Al_(x)Ga_(y)In_(z)N wafer has a root mean        square (RMS) surface roughness of less than 1 nm in a 10×10 μm²        area at such second side thereof.

In one embodiment of such fabrication method, the first side of thewafer is the N-side of a GaN wafer and the second side is the Ga-side ofsuch GaN wafer.

The Al_(x)Ga_(y)In_(z)N wafer blank as used in such fabrication methodmay be produced in any suitable manner, as for example: (1) growing anAl_(x)Ga_(y)In_(z)N boule and then slicing it into wafer blanks; or (2)growing a thick Al_(x)Ga_(y)In_(z)N film on a foreign substrate and thenseparating such thick film from the substrate. The wafer blank may beoriented so that the c-axis is perpendicular to the wafer surface or itmay be intentionally slightly Disoriented (c-axis not perpendicular tothe wafer surface) to facilitate subsequent epitaxial growth, deviceprocessing or device design.

The Al_(x)Ga_(y)In_(z)N wafer blank may be subjected to processing forreducing the internal stress caused, for example, by the disparity ofthermal coefficients and lattice constants between suchAl_(x)Ga_(y)In_(z)N wafer and the foreign substrate on which it isgrown. Reduction of internal stress may be conducted by any suitabletechnique, e.g., by thermally annealing the Al_(x)Ga_(y)In_(z)N wafer orchemically etching the wafer.

Thermal annealing can be carried out at elevated temperature conditions,e.g., from about 700° C. to about 1000° C., at appropriate pressure(which in various embodiments may be atmospheric pressure,sub-atmospheric pressure, or superatmospheric pressure), in nitrogen,ammonia or any other suitable environment, for sufficient time to effectthe desired nature and extent of thermal annealing, e.g., a time of fromabout 1 minute to about 1 hour in some embodiments, or greater periodsin other embodiments, with the choice of specific annealing processconditions being readily determinable by simple experiment involvingvariation of process conditions and characterization of the annealedproduct.

Chemical etching of the Al_(x)Ga_(y)In_(z)N wafer functions to remove alayer of surface material from said wafer, thereby relaxing the internalstress of said wafer. It is preferred that the chemical etching processeffect a removal of surface material of less than 100 μm thickness fromthe wafer, and more preferably less than 10 μm thickness.

The Al_(x)Ga_(y)In_(z)N wafer can be chemically etched at elevatedtemperature by a very strong acid, e.g., sulfuric acid, phosphoric acid,combinations thereof, etc., or by a very strong base, e.g., molten KOHor NaOH.

Lapping slurry compositions advantageously used in the practice of thepresent invention can comprise any suitable abrasives, including, butnot limited to, diamond powders, silicon carbide powders, boron carbidepowders, and alumina powders. Preferably, the lapping slurry comprisesdiamond powder having an average particle size in a range of from about6 μm to about 10 μm. More preferably, two or more lapping slurries lapthe Al_(x)Ga_(y)In_(z)N wafer blank, with each subsequent lapping slurrycomprising abrasives of a progressively smaller average particle size.For example, the Al_(x)Ga_(y)In_(z)N wafer blank may be lapped by afirst slurry comprising abrasives of an average particle size of fromabout 8 μm to about 10 μm, and then by a second slurry comprisingabrasives of an average particle size of from about 5 μm to about 7 μm.

Similarly, mechanical polishing slurries useful in the practice of thepresent invention may comprise any suitable abrasives, including but notlimited to diamond powders, silicon carbide powders, boron carbidepowders, and alumina powders. Diamond powders with an average particlesize in a range of from about 0.1 μm to about 3 μm are particularlypreferred. The mechanical polishing step may also employ two or moremechanical polishing slurries, with each subsequent mechanical polishingslurry comprising abrasives of a progressively smaller average particlesize. For example, a first mechanical polishing slurry comprisingabrasives of an average particle size of from about 2.5 μm to about 3.5μm can be used, followed by a second mechanical polishing slurrycomprising abrasives of an average particle size of from about 0.75 μmto about 1.25 μm, followed by a third mechanical polishing slurrycomprising abrasives of an average particle size of from about 0.35 μmto about 0.65 μm, followed by a fourth mechanical polishing slurrycomprising abrasives of an average particle size of from about 0.2 μm toabout 0.3 μm, and finally by a fifth mechanical polishing slurrycomprising abrasives of an average particle size of from about 0.1 μm toabout 0.2 μm.

The CMP slurry comprises at least one chemical reactant, e.g., an acidor a base in an amount yielding a pH in a range of 0≦pH<7 for acidic CMPslurry compositions, or a pH in a range of 7<pH≦14 for basic CMP slurrycompositions.

After the CMP, the Al_(x)Ga_(y)In_(z)N wafer may be subjected toadditional processing for further reducing the stress of the wafer andimproving the surface quality. A mild etching may be effective for thispurpose. The mild etching may for example remove some residual surfacedamage on a Ga-side surface resulting from final CMP polishing while notetching the undamaged surface of such Ga-side, thereby improving thesurface quality of the wafer. The mild etching can also remove thedamage on an N-side surface, thus reduce the stress on the wafer causedby surface damage. Mild etching can also be employed to produce a mattefinish on the N-side surface. For example, the wafer can be slightlyetched in an aqueous solution of base (for example, KOH or NaOH) or anaqueous solution of acid (for example, HF, H₂SO₄, or H₃PO₄) at atemperature below the boiling point of the aqueous solution, typicallyabout 100° C.

In a further aspect, the invention relates to a method of fabricating alaser facet on an article formed of Al_(x)Ga_(y)In_(z)N, wherein 0≦x≦1,0≦y≦1, 0≦z≦1, and x+y+z=1. The method comprises chemically mechanicallypolishing the article at a surface thereof using a chemical mechanicalpolishing slurry including silica and/or alumina abrasive particles, andan acid or base, wherein the chemically mechanically polishing step iscarried out to impart to the surface a root mean square (RMS) surfaceroughness of less than 1 nm in a 10×10 μm² area thereof.

A still further aspect of the invention relates to chemicallymechanically polishing Al_(x)Ga_(y)In_(z)N, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1,and x+y+z=1, using a chemical mechanical polishing slurry includingsilica and/or alumina abrasive particles, and an acid or base, whereinthe chemically mechanically polishing step is carried out to impart tothe surface a root mean square (RMS) surface roughness of less than 1 nmin a 10×10 μm² area thereof. Such methodology may be employed forprocessing of Al_(x)Ga_(y)In_(z)N wafers, e.g., in the fabrication ofsuch wafers in a wafer fab, for planarization of such wafers, or forreplanarization of such wafers, reworking of wafers to remove undesiredlayers or material therefrom, etc.

The term “Al_(x)Ga_(y)In_(z)N” as used herein per se is intended to bebroadly construed as including all compositions wherein 0≦x≦1, 0≦y≦1 and0≦z≦1 and x+y+z=1, and therefore is inclusive, inter alia, of AlN,AlGaN, AlInN, AlGaInN, GaN, GaInN, and InN.

Other aspects, features, and embodiments of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a Nomarski optical micrograph (×100) of a GaN surface afterbeing mechanically polished with 1 μm diamond slurry until mirror finishwas achieved.

FIG. 2 is an AFM image of the GaN surface shown in FIG. 1.

FIG. 3 is a Nomarski optical micrograph (×100) of a GaN surface afterbeing chemically mechanically polished with acidic colloidal silica CMPslurry (pH=0.8) for 1 hour and cleaned in diluted hydrofluoric acid.

FIG. 4 is an atomic force microscopy (AFM) image of the GaN surfaceshown in FIG. 3.

FIG. 5 is an AFM image of a GaN surface after being chemicallymechanically polished with acidic colloidal alumina CMP slurry (pH=3.6)comprising hydrogen peroxide as oxidization agent for 1 hour and cleanedwith diluted hydrofluoric acid.

FIG. 6 is a Nomarski optical micrograph (×100) of a GaN surface afterbeing chemically mechanically polished with basic colloidal silica CMPslurry (pH=11.2) for 1 hour and cleaned in diluted hydrofluoric acid.

FIG. 7 is an AFM image of the GaN surface shown in FIG. 6.

FIG. 8 is an AFM image of a GaN surface after being chemicallymechanically polished with acidic silica CMP slurry (pH=0.8) for 1 hourand cleaned in diluted hydrofluoric acid.

FIG. 9 is a scanning electron microscopy (SEM) micrograph of a GaNsurface after being chemically mechanically polished with acidic silicaCMP slurry (pH=0.8) for 1 hour and cleaned in diluted hydrofluoric acid.

FIG. 10 is an AFM image of a CMP-polished GaN sample (50×50 μm) at asurface parallel to the c-axis of the GaN.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

Fabrication of high quality Al_(x)Ga_(y)In_(z)N wafers in accordancewith a preferred aspect of the present invention is readily achieved byprocessing steps as hereinafter more fully described, includingfabrication of freestanding Al_(x)Ga_(y)In_(z)N wafer blanks, lapping,mechanical polishing, chemical mechanical polishing, and reduction ofinternal stress.

Freestanding Al_(x)Ga_(y)In_(z)N wafer blanks are obtained by any ofvarious suitable methods. One method involves first growing anAl_(x)Ga_(y)In_(z)N boule and then slicing it into wafer blanks. Anothermethod for producing Al_(x)Ga_(y)In_(z)N wafer blanks utilizes the stepsof: (1) growing a thick Al_(x)Ga_(y)In_(z)N film on a foreign substrate,using a suitable method such as hydride vapor phase epitaxy (HVPE), themetallorganic chloride (MOC) method, metallorganic chemical vapordeposition (MOCVD), sublimation, etc.; and then (2) removing the foreignsubstrate from the thick Al_(x)Ga_(y)In_(z)N film, by polishing oretching the foreign substrate, by a laser-induced liftoff process, orother suitable technique.

By way of example, GaN films of about 400 μm thickness can be grown onsapphire substrates using HVPE process techniques.

Marks such as flats are made on the wafer to identify the crystalorientation of such wafer blank. The Al_(x)Ga_(y)In_(z)N wafer blank canbe sized into a round shape by, for example, particle beams, tofacilitate subsequent mounting or processing of the wafer blank.

Mounting of the freestanding Al_(x)Ga_(y)In_(z)N wafer blank to afixture enables it to be readily lapped or polished as necessary. Thewafer blank can be mounted on a template with recesses for holding thewafer blank. Alternatively, the wafer blank can be mounted on a flattemplate, by, for example, (1) heating such template on a hotplate, (2)applying wax onto such template, and (3) pressing the wafer blankagainst the waxed template. After the template cools down, the waxsolidifies and functions to hold the wafer blank on the template.

When the Al_(x)Ga_(y)In_(z)N wafer blank is obtained from aAl_(x)Ga_(y)In_(z)N boule and is relatively thick and uniform, arecessed template can be used for mounting such wafer blank, which isadvantageous over waxed templates in respect of shorter process times,easier demounting, and reduced contamination.

On the other hand, for Al_(x)Ga_(y)In_(z)N wafer blanks which may bemore fragile, thinner, or less uniform in thickness, for example, waferblanks obtained from HVPE processes, the use of recessed templates maybe less preferred due to the associated risk of breaking theAl_(x)Ga_(y)In_(z)N wafer during the lapping and/or polishing process.

The fixture used for mounting the Al_(x)Ga_(y)In_(z)N wafer blank can beof any suitable type appropriate to, and compatible with, the respectivelapping or polishing apparatus. For the purpose of improving thicknessuniformity of the Al_(x)Ga_(y)In_(z)N wafer, a special lapping fixturecomprising three adjustable diamond stops defining a plane can beutilized. The plane defined by the stops is parallel to the fixturesurface, at a predetermined distance away from the surface. Suchpredetermined distance defines a minimum thickness of the lappedAl_(x)Ga_(y)In_(z)N wafer, because the three diamond stops function asstop points preventing further removal of surface material from theAl_(x)Ga_(y)In_(z)N wafer.

In case the Al_(x)Ga_(y)In_(z)N wafer blank is slightly bowed orotherwise distorted due to internal stress present therein, it ispreferable to dispose a weight on the wafer blank during the wafer beingwax-mounted on the template. The type and amount of weight for suchpurpose is readily determinable within the skill of the art.

After the Al_(x)Ga_(y)In_(z)N wafer blank is appropriately mounted, thewafer blank can be lapped by pressing it against a lapping plate, withabrasive particles embedded on surface of such lap plate, to produce aflat surface on the wafer. The pressure on the wafer may be adjusted tocontrol the lapping process.

When using the same abrasives and lap plate rotation rates, the lappingrates of the Al_(x)Ga_(y)In_(z)N wafer blank increase with increasingparticle size of the abrasive. Larger abrasive particles thus result ina higher lapping rate, but produce rougher lapped surfaces.

Lapping rates also depend on the hardness of abrasive material used. Forexample, diamond powders have higher lapping rates than silicon carbidepowders, which in turn have higher lapping rates than alumina powders.

Lapping rates also depend on the type of lapping plates employed. Forexample, a copper lapping plate has a lower lapping rate than that of acast iron plate, but the copper lapping plate yields a smoother lappedsurface than that produced by the cast iron plate.

For an optimal lapping result, many factors, such as process time,surface finish, and manufacturing cost, have to be considered, and manycombinations of abrasive material, particle size, lapping rate, andwafer pressure can be employed in the practice of the present invention.In order to reduce the probability of Al_(x)Ga_(y)In_(z)N wafercracking, a pressure below 5 psi, preferably 2 psi, is preferred. Inorder to reduce process time, a lapping rate above 50 μm/hr is preferredfor stock removal. Among various kinds of abrasive materials, such asdiamond, silicon carbide, boron carbide, and alumina, diamond slurry ispreferred due to its high material removal rate and its production ofbetter surface finishes.

Lapping of the Al_(x)Ga_(y)In_(z)N wafer blank can be achieved either bya single step, or by multiple steps, with each subsequent lapping stepusing abrasives of progressively smaller particle sizes. After eachlapping step, an optical microscope can be used to examine the surfaceto make sure that surface damage from previous steps has beensubstantially removed, before proceeding to next step.

In one illustrative embodiment of the invention, a single lapping slurryis used, comprising 9 μm diamond abrasive, for lapping anAl_(x)Ga_(y)In_(z)N wafer on a cast iron lapping plate under a pressureof 1 psi. The size of diamond abrasive particles is provided by thediamond slurry manufacturer, and is the average size of diamondparticles in the slurry.

In another illustrative embodiment of the invention, two lappingslurries are used: the first lapping slurry comprises 9 μm diamondabrasive for lapping an Al_(x)Ga_(y)In_(z)N wafer on a cast iron lappingplate, and the second slurry comprises 6 μm diamond abrasive for lappingthe same wafer on a copper plate to achieve the desired surface finish.

After the Al_(x)Ga_(y)In_(z)N wafer is lapped, it can be mechanicallypolished to achieve smooth surface morphology. During the mechanicalpolishing process, the Al_(x)Ga_(y)In_(z)N wafer is pressed against apolishing pad with abrasive particles. Polishing processes typicallyyield better surface finish than lapping, even with a same size diamondslurry. Polishing can be achieved either by a single step, or bymultiple steps, with each subsequent polishing step using abrasives ofprogressively smaller particle sizes.

After the mechanical polishing process, the Al_(x)Ga_(y)In_(z)N wafersurface becomes relatively smooth. FIG. 1 shows a Nomarski opticalmicrograph (×100) of a GaN surface after being mechanically polishedwith 1 μm diamond slurry until mirror finish has been achieved. However,such Al_(x)Ga_(y)In_(z)N wafer is not suitable for homoepitaxial growthof Al_(x)Ga_(y)In_(z)N crystals, since it still has significant surfaceand subsurface damage. The surface damage is characterized by densepolishing scratches that are visible under the atomic force microscope(AFM), as shown in FIG. 2.

To remove such surface and subsurface damage and polishing scratches,chemical mechanical polishing (CMP) of the Al_(x)Ga_(y)In_(z)N wafer iscarried out.

A first CMP slurry effective for chemically mechanically polishing theGa-side of the Al_(x)Ga_(y)In_(z)N wafer comprises an acid and abrasiveamorphous silica particles, such as fumed silica or colloidal silica,having particle sizes of less than 200 nm. The pH value of such CMPslurry may for example be in a range from about 0.5 to about 4. In someapplications, it may be advantageous to employ a CMP slurry thatincludes an oxidization agent, such as hydrogen peroxide,dichloroisocyanuric acid or the like.

FIGS. 3 and 4 show a Nomarski optical micrograph and an AFM image,respectively, of a GaN wafer chemically mechanically polished using anacidic colloidal silica slurry having a pH value of 0.8 for about 1hour. The GaN wafer was first polished with 1 μm diamond slurry beforeCMP. Besides a few defects from the substrate, the GaN surface is verysmooth, with RMS surface roughness of about 0.15 nm in a 2×2 μm² areaand about 0.5 nm in a 10×10 μm² area. Further, a previously unseen stepstructure is observed on the GaN surface under AFM. The presence of suchstep structure is an indication that the CMP process has been successfulin removing polishing scratches from previous mechanical polishing. TheCMP rate using such slurry can for example be on the order of about 2μm/hr.

To further ascertain that the CMP process has also removed thesubsurface damage on the surface, the wafer after CMP processing isetched with a strong etchant, H₃PO₄ at 180° C. for 5 minutes. At thisetching condition, crystal defects as well as surface and subsurfacedamage on the Ga-side of GaN surface will be etched at a greater ratethan good crystalline material, producing etching pits. The size andnumber of the pits can be studied with an atomic force microscope. Afterhot H₃PO₄ etching, the CMP polished wafers show some etching pits, butthe density of the etching pits is the same as the density of pitsevident in the CMP polished surface. The size of the pits has increased,however. For comparison, a wafer that is not completely polished withthe CMP process (i.e., using a shorter CMP processing time resulting insome residual polishing damage) shows more etching pits after etchingwith H₃PO₄ at 180° C. for 5 minutes, and many of the pits follow a line,indicating that the surface damage and subsurface damage are notcompletely removed when the CMP process is not complete.

Oxidation agents may optionally and advantageously be employed in theacidic CMP slurry, as necessary or desired in specific applications ofsuch CMP methodology of the invention. When hydrogen peroxide ordichloroisocyanuric acid is used as an oxidation agent, the polishingrate is above 2 μm/hr, with RMS surface roughness being below 0.2 in a2×2 μm² area and below 0.5 nm in a 10×10 μm² area. The step structureson the Al_(x)Ga_(y)In_(z)N wafer surface are readily observed by AFMinspection.

A second CMP slurry effective for chemically mechanically polishing theGa-side of the Al_(x)Ga_(y)In_(z)N wafer comprises an acid and abrasivecolloidal alumina particles having particle sizes of less than 200 nm.The pH value of such CMP slurry in one preferred embodiment is in arange from about 3 to about 4. Optionally, such CMP slurry can alsocomprise an oxidization agent, such as hydrogen peroxide,dichloroisocyanuric acid or the like.

FIG. 5 shows an AFM image of a GaN surface after being chemicallymechanically polished with acidic colloidal alumina CMP slurry (pH=3.6)comprising hydrogen peroxide as an oxidization agent, for 1 hour. Thestep structure is observed under AFM, demonstrating that acidiccolloidal alumina slurry is effective for removing mechanical damagefrom the GaN surface. However, at the same polishing operationconditions, the colloidal alumina-based slurry has a much lowerpolishing rate (about 0.1 μm/hr) than the polishing rate of thesilica-based slurries. Because of such slow polishing rate, manypolishing scratches are still present after 1 hour of polishing with theacidic colloidal alumina CMP slurry. A much longer polishing time isneeded to completely remove the surface/subsurface damage with thecolloidal alumina-based slurry than with the silica-based slurries.

A third CMP slurry effective for chemically mechanically polishing theGa-side of the Al_(x)Ga_(y)In_(z)N wafer comprises a base and amorphoussilica particles, either fumed silica or colloidal silica, havingparticle sizes of less than 200 nm. The pH value of such illustrativeCMP slurry is in a range of from about 8 to about 13.5.

FIGS. 6 and 7 show a Nomarski optical micrograph and an AFM image of aGaN wafer chemically mechanically polished using a basic colloidalsilica slurry having a pH value of 11.2, for about 1 hour. The surfaceappears rougher and has significantly more scratches when polished, incomparison with the surface finish achieved with an acidic silicaslurry. Moreover, the scratches are larger and deeper than those of theGaN surface after mechanical polishing with diamond slurry comprising 1μm diamond powders, indicating that larger particles or particleagglomerations are present in the basic silica slurry. Interestingly,step structures are also observed. The presence of step structuresindicates that surface damage from previous mechanical polishing hasbeen removed, but the presence of larger particles in the slurryintroduces new damage. It therefore is desirable to filter the basicsilica slurry before polishing to remove large particles and to ensurethat the abrasive particles in such slurry have particle sizes of lessthan 200 nm.

Besides using hydroxides for pH alteration, the pH of the basic silicaslurry can be adjusted with ammonia or alkanolamine. Ammonia- oralkanolamine-stabilized slurries provide smoother polished surfaces andtherefore are preferred over hydroxide-based slurries.

To improve the stability of the CMP process, it may be advantageous tocontrol the ambient humidity and temperature during the CMP process.

After chemical mechanical polishing, the Al_(x)Ga_(y)In_(z)N wafer canbe cleaned and dried, using techniques known in the art. A mild etchingcan also be used to remove any remaining surface and subsurface damagefrom the final polished wafer. The condition for the mild etching ischosen to remove some residual surface damage on the Ga-side surfacefrom final polishing, while not etching or etching to a limited degreethe undamaged surface of the Ga-side. The mild etching can also removethe damage on the N-side surface to reduce the stress on the wafercaused by damage on the N-surface. This mild etching can also produce amatte finish on the N-surface. For example, the wafer can be slightlyetched in an aqueous solution of base (for example, KOH or NaOH) or anaqueous solution of acid (for example, HF, H₂SO₄, or H₃PO₄) at atemperature below 100° C.

Al_(x)Ga_(y)In_(z)N wafers may suffer from internal stresses, whichcause the wafer to bow or to warp. Thermal annealing or chemical etchingof the Al_(x)Ga_(y)In_(z)N wafer, which can be performed before, afteror between the steps of the wafer fabrication sequence, can relax suchinternal stresses.

In the circumstance where the Al_(x)Ga_(y)In_(z)N wafer has large pitson its surface and contaminants are trapped in the pits from thefabrication process, it is beneficial to employ a chemical etching andcleaning step to remove the contaminants from the pits between the stepsof wafer fabrication.

In one embodiment of the present invention, the Al_(x)Ga_(y)In_(z)Nwafer is subjected to thermal annealing at temperature up to 1000° C. ina nitrogen ambient. Preferably, the annealing temperature is in a rangeof from about 700° C. to about 1000° C., and the duration of the thermalannealing is in a range of from about 1 minute to about 1 hour.

In another embodiment of the invention, the Al_(x)Ga_(y)In_(z)N wafer issubjected to chemical etching, which preferentially removes damagedsurface material from the Al_(x)Ga_(y)In_(z)N wafer and reduces waferbow and warp caused by surface damage.

Chemical etching of the Al_(x)Ga_(y)In_(z)N wafer can be accomplished byimmersing the wafer in very strong acids or bases at an elevatedtemperature. Sulfuric acid or phosphoric acid at a temperature above150° C. can be utilized to etch the Al_(x)Ga_(y)In_(z)N wafer.Alternatively, molten potassium or sodium hydroxide can also be employedto etch the Al_(x)Ga_(y)In_(z)N wafer. The etching conditions, such asetching temperature and etching time, are preferably controlled to yieldremoval of surface material of less than 100 μm thickness, andpreferably less than 10 μm thickness.

After chemical mechanical polishing of a GaN surface, for example, usingacidic silica CMP slurry (pH=0.8) for about 1 hr., small pits areformed, which may originate from dislocations in the crystal lattice ofthe GaN wafer. The diameter of the pits is typically below 1 μm, andmore typically below 0.5 μm. The pits appear round without clear edgeswhen imaged with an atomic force microscope. When the wafer has beencompletely CMP polished and is subjected to etching, for example, withH₃PO₄ at 180° C. for 5 minutes, the size of the pits is increased, butthe density of the pits remain the same, i.e., no more pits areproduced. Furthermore, the pits formed from etching the CMP polishedwafer appear hexagonal when imaged with an atomic force microscope.

FIG. 8 shows an AFM image of a GaN surface, with clearly visible pits.The GaN surface was chemical mechanically polished using an acidiccolloidal silica CMP slurry (pH=0.8) for about 1 hour.

FIG. 9 also shows a scanning electron microscopic (SEM) image of a GaNwafer, polished by acidic colloidal silica CMP slurry (pH=0.8) for 1hour, with visible pits that can be counted for determining the defectdensity of such GaN wafer. Without chemical mechanical polishing of theGaN wafer surface, such pits are not observed with AFM or with SEM.

In one aspect of the present invention, a CMP process is used to preparean Al_(x)Ga_(y)In_(z)N wafer, to highlight crystal defects forsubsequent determination of defect density by AFM or SEM techniques.

This defect highlight technique is superior to other techniques such astransmission electron microscope (TEM), wet-chemical etching, and photoelectrochemical etching, which are generally conducted under harshetching conditions, making the etched Al_(x)Ga_(y)In_(z)N waferunsuitable for subsequent epitaxial growth of Al_(x)Ga_(y)In_(z)Ncrystalline material thereon.

By contrast, the use of a CMP process for highlighting crystal defectsdoes not damage the crystal surface of the Al_(x)Ga_(y)In_(z)N wafer andtherefore permits subsequent crystal growth.

From the foregoing, it will be apparent that the present inventionprovides a superior technique for chemically mechanically polishingAl_(x)Ga_(y)In_(z)N, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1, using achemical mechanical polishing slurry including silica and/or aluminaabrasive particles, and an acid or base, wherein the chemicallymechanically polishing step is carried out to impart to the surface aroot mean square (RMS) surface roughness of less than 1 nm in a 10×10 μmarea thereof.

Such methodology may be employed for processing of Al_(x)Ga_(y)In_(z)Nwafers, e.g., in the fabrication of such wafers in a wafer fab, forplanarization of such wafers, or for replanarization of such wafers,reworking of wafers to remove undesired layers or material therefrom,etc., as well as in fabricating laser facets on Al_(x)Ga_(y)In_(z)N fordevice applications.

The method of the invention for chemically mechanically polishingAl_(x)Ga_(y)In_(z)N may also be employed for shaping ofAl_(x)Ga_(y)In_(z)N articles to counteract bow or other deformation ormis-shaping, such as otherwise may be manifested in the processing ofsuch Al_(x)Ga_(y)In_(z)N articles.

EXAMPLE 1

A GaN film several hundred microns thick was grown on a sapphiresubstrate by an HVPE process and then separated from the sapphiresubstrate. The resultantly formed freestanding GaN wafer blank exhibiteda textured Ga-surface with a RMS roughness of about 4 nm in a 2×2 μmarea.

The GaN wafer blank was then polished at the Ga-side with an acidicsilica slurry without undergoing a lapping process.

After polishing, it was observed that the surface morphology of such GaNwafer was greatly improved, with the textured surface being entirelyremoved. The RMS roughness was reduced to below 0.3 nm in a 2×2 μm²area.

EXAMPLE 2

Thick GaN films with thickness in the range from 200-500 microns weregrown on 2″ sapphire substrates by an HVPE process. The GaN films thenwere separated from the sapphire substrate, to yield freestanding GaNwafer blanks.

Flats for the GaN films were marked as 30° off the sapphire substrate'sflat. The GaN wafer blanks then were sized into wafer shapes withdiameter of 30, 35, and 40 mm using a particle beam jet. To preventwafer breakage during wafer sizing, it was preferable to mount the GaNwafer on a glass plate of at least 1 mm thickness, using wax.

Nine GaN wafers were mounted on a lap fixture with wax with the N-sidefacing the lap fixture. A steel block was placed on top of each waferwhile the wax cooled. The GaN wafers were first lapped on the Ga-sidewith diamond slurry of 9 μm in diameter on a cast iron lapping plate.Before lapping, a large thickness variation existed between the wafersand within each wafer. After lapping, uniformity of wafer thickness wasgreatly improved.

The wafers then were removed from the lapping fixture, and wax-mountedon a mechanical polishing fixture. Each wafer was polished with diamondslurry of 3 μm in diameter until mirror finish was achieved. Underoptical microscope examination, all surface damage from the lappingprocess was removed.

After mechanical polishing, the wafers were chemically mechanicallypolished with acidic colloidal silica slurry. A Nomarski opticalmicroscope was used to examine the polished surfaces, to verify that theCMP process removed all mechanical polishing scratches.

EXAMPLE 3

Three GaN wafer blanks were mounted on a lap fixture with wax with theGa-side facing the lap fixture. A steel block was placed on top of eachwafer while the wax cooled. The GaN wafers were first lapped on theN-side with diamond slurry of 9 μm in diameter on a Lapmaster 15 lappingmachine with cast iron lapping plate until a uniform mat finish wasachieved.

After the N-side was lapped, the GaN wafers were removed from the lapfixture by heating on a hot plate. The wafers were cleaned and mountedon a lap fixture with wax with the N-side facing the lap fixture. Asteel block was placed on top of each wafer while the wax cooled. TheGaN wafers were lapped on the Ga-side with diamond slurry of 9 μm indiameter on a cast iron lapping plate until a desirable wafer thicknesswas obtained. Subsequently, the GaN wafers were lapped with diamondslurry of 6 μm in diameter on a copper lapping plate until surfacefeatures from the previous lapping step were removed.

After lapping, the three wafers were mechanically polished with diamondslurry of 1 μm in diameter on a Buehler ECOMET polisher until thesurface features from the previous lapping step were removed.

After mechanical polishing, the three wafers were chemical mechanicallypolished with an acidic colloidal silica slurry on a Beuhler ECOMETpolisher. The acidic colloidal silica slurry was prepared by mixing 2parts of 1 molar aqueous hydrochloric acid with 1 part of commercialsilica slurry, Nalco 2350 polishing slurry. A Nomarski opticalmicroscope was used to examine the polished surfaces, to verify that theCMP process removed all mechanical polishing scratches.

After CMP process, the wafers were removed from the polish fixture andcleaned. The wafers were also cleaned in diluted hydrofluoric acid toremove any residual colloidal silica particles on the wafer surface. Thewafers were imaged with atomic force microscope (Digital InstrumentsNanoScope III) to determine the density of the pits and the smoothnessof the surface. For one wafer, the RMS roughness was 0.11 nm in a 2×2μm² area and 0.28 nm in a 10×10 μm area. The pit density for the threewafers was about 10⁶-10⁷ pits/cm², and the pit size was about less than0.4 μm in diameter.

EXAMPLE 4

A GaN boule was sliced vertically, to produce wafer samples withsurfaces that were parallel to the c-axis.

The sliced samples were first lapped with 9 μm diamond slurry on a castiron plate with a Lapmaster 15 lapping machine to remove the wire sawmarks. Subsequently, the samples were polished with 3 μm diamond slurryon a Suba500 polishing pad to obtain a mirror finish. The surfaces stillhad scratches that were observed under an optical microscope.

The samples next were chemical mechanically polished with a chemicalmechanical polishing (CMP) slurry consisting of a mixture of colloidalsilica and hydrochloric acid, for 1 hour. Such CMP processing removedthe surface scratches from the samples.

FIG. 10 is an AFM image of a CMP-polished GaN sample (50×50 μm) at asurface parallel to the c-axis of the GaN, after such processing.

The white spots on the sample surface were residual CMP slurryparticles. The CMP polished surface was very smooth, and was determinedto have a RMS roughness of 0.8 nm over the 50 μm×50 μm area.Furthermore, no polish scratches were observed, indicating that the CMPslurry was effective in removing the surface damage of this GaN sampleon the surface parallel to the c-axis.

The GaN wafers of the present invention can be used to constructoptoelectronic devices such as light emitting diodes and blue lightlasers. Such devices are important, as the blue light emitting diodes(LEDs) and lasers are an enabling technology, allowing much higherstorage density in magneto-optic memories and CDROMs and theconstruction of full color light emitting displays. Such devices canreplace today's incandescent light bulbs in road and railway signals,etc., where they promise very substantial cost and energy savings.

The invention has been described herein with reference to specificfeatures, aspects, and embodiments. It will be appreciated that theapplicability of the invention is not thus limited, but readily extendsto and encompasses numerous variations, modifications, and otherembodiments, as will readily suggest themselves to those of ordinaryskill in the art. Accordingly, the invention is to be broadly construed,consistent with the claims hereafter set forth.

1. A wafer comprising Al_(x)Ga_(y)In_(z)N, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1and x+y+z=1, said wafer having a Ga-side surface characterized by a rootmean square (RMS) surface roughness of less than 1 nm in a 10×10 μm²area and having a pit density of about 10⁶ to about 10⁷ pits/cm².
 2. Thewafer according to claim 1, wherein pits constituting said pit densityare less than 0.4 μm in diameter.
 3. The wafer of claim 1, wherein theRMS surface roughness of said wafer is less than 0.7 nm in a 10×10 μm²area.
 4. The wafer of claim 1, wherein the RMS surface roughness of saidwafer is less than 0.5 nm in a 10×10 m² area.
 5. The wafer of claim 1,wherein the RMS surface roughness of said wafer is at least less than0.4 nm in a 2×2 μm² area.
 6. The wafer of claim 1, wherein the RMSsurface roughness of said wafer is less than 0.2 nm in a 2×2 μm² area.7. The wafer of claim 1, wherein the RMS surface roughness of said waferis less than 0.15 nm in a 2×2 m² area.
 8. The wafer of claim 1,characterized by a step structure when observed with an atomic forcemicroscope.
 9. The wafer of claim 1, wherein the crystal defects arevisible as small pits with diameters of less than 1 μm.
 10. The wafer ofclaim 1, formed by chemically mechanically polishing (CMP) anAl_(x)Ga_(y)In_(z)N wafer blank, using a silica- or alumina-containingCMP slurry.
 11. An epitaxial Al_(x)Ga_(y)In_(z)N crystal structure,comprising an epitaxial Al_(x)Ga_(y)In_(z)N thin film grown on a wafercomprising Al_(x)Ga_(y)In_(z)N, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1 and x+y+z=1,said wafer having a Ga-side surface characterized by a root mean square(RMS) surface roughness of less than 1 nm in a 10×10 μm area and havinga pit density of about 10⁶ to about 10⁷ pits/cm².
 12. The epitaxialAl_(x)Ga_(y)In_(z)N crystal structure of claim 11, comprising a wurtzitecrystalline thin film.
 13. The epitaxial Al_(x)Ga_(y)In_(z)N crystalstructure of claim 11, where the epitaxial Al_(x′)Ga_(y′)In_(z′)N thinfilm has the same composition as the wafer comprisingAl_(x)Ga_(y)In_(z)N.
 14. The epitaxial Al_(x)Ga_(y)In_(z)N crystalstructure of claim 11, where the epitaxial Al_(x′)Ga_(y′)In_(z′)N thinfilm has a different composition from the wafer comprisingAl_(x)Ga_(y)In_(z)N.
 15. The epitaxial Al_(x)Ga_(y)In_(z)N crystalstructure of claim 11, where the epitaxial Al_(x′)Ga_(y′)In_(z′)N thinfilm has a graded composition.
 16. An optoelectronic device comprisingat least one epitaxial Al_(x′)Ga_(y′)In_(z′)N crystal structure grown ona wafer comprising Al_(x)Ga_(y)In_(z)N, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1 andx+y+z=1, said wafer having a Ga-side surface characterized by a rootmean square (RMS) surface roughness of less than 1 nm in a 10×10 μm²area and having a pit density of about 10⁶ to about 10⁷ pits/cm². 17.The optoelectronic device of claim 16, wherein the optoelectronic deviceis a light emitting diode.
 18. The optoelectronic device of claim 16,wherein the optoelectronic device is a blue light laser diode.
 19. Theoptoelectronic device of claim 16, wherein the optoelectronic device isincorporated into a light emitting diode.
 20. The optoelectronic deviceof claim 16, wherein the optoelectronic device is incorporated into amagneto-optic memory device.
 21. The optoelectronic device of claim 16,wherein the optoelectronic device is incorporated into a full colorlight emitting display.
 22. The optoelectronic device of claim 16,wherein the optoelectronic device is incorporated into a DVD device. 23.A microelectronic device comprising at least one epitaxialAl_(x′)Ga_(y′)In_(z′)N crystal structure grown on a wafer comprisingAl_(x)Ga_(y)In_(z)N, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1 and x+y+z=1, said waferhaving a Ga-side surface characterized by a root mean square (RMS)surface roughness of less than 1 nm in a 10×10 μm² area and having a pitdensity of about 10⁶ to about 10⁷ pits/cm².
 24. An epitaxialAl_(x′)Ga_(y′)In_(z′)N crystal boule grown on a wafer comprisingAl_(x)Ga_(y)In_(z)N, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1 and x+y+z=1, said waferhaving a Ga-side surface characterized by a root mean square (RMS)surface roughness of less than 1 nm in a 10×10 μm² area and having a pitdensity of about 10⁶ to about 10⁷ pits/cm².
 25. Al_(x)Ga_(y)In_(z)N,wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1, having a Ga-side surfacecharacterized by a root mean square (RMS) surface roughness of less than1 nm in a 10×10 μm² area and having a pit density of about 10⁶ to about10⁷ pits/cm².
 26. GaN, having a Ga-side surface characterized by a rootmean square (RMS) surface roughness of less than 1 nm in a 10×10 μm²area and having a pit density of about 10⁶ to about 10⁷ pits/cm².